by them that have been conversant with Microscopes. And it may be also
tried by applying any substance to a hole through which some Light is
immitted into a dark Room. For how opake soever that Substance may seem
in the open Air, it will by that means appear very manifestly
transparent, if it be of a sufficient thinness. Only white metalline
Bodies must be excepted, which by reason of their excessive density seem
to reflect almost all the Light incident on their first Superficies;
unless by solution in Menstruums they be reduced into very small
Particles, and then they become transparent.
PROP. III.
_Between the parts of opake and colour'd Bodies are many Spaces, either
empty, or replenish'd with Mediums of other Densities; as Water between
the tinging Corpuscles wherewith any Liquor is impregnated, Air between
the aqueous Globules that constitute Clouds or Mists; and for the most
part Spaces void of both Air and Water, but yet perhaps not wholly void
of all Substance, between the parts of hard Bodies._
The truth of this is evinced by the two precedent Propositions: For by
the second Proposition there are many Reflexions made by the internal
parts of Bodies, which, by the first Proposition, would not happen if
the parts of those Bodies were continued without any such Interstices
between them; because Reflexions are caused only in Superficies, which
intercede Mediums of a differing density, by _Prop._ 1.
But farther, that this discontinuity of parts is the principal Cause of
the opacity of Bodies, will appear by considering, that opake Substances
become transparent by filling their Pores with any Substance of equal or
almost equal density with their parts. Thus Paper dipped in Water or
Oil, the _Oculus Mundi_ Stone steep'd in Water, Linnen Cloth oiled or
varnish'd, and many other Substances soaked in such Liquors as will
intimately pervade their little Pores, become by that means more
transparent than otherwise; so, on the contrary, the most transparent
Substances, may, by evacuating their Pores, or separating their parts,
be render'd sufficiently opake; as Salts or wet Paper, or the _Oculus
Mundi_ Stone by being dried, Horn by being scraped, Glass by being
reduced to Powder, or otherwise flawed; Turpentine by being stirred
about with Water till they mix imperfectly, and Water by being form'd
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